The present invention relates to a technology capable of measuring efficiently and in high precision a lattice constant of a specimen having a multi-layer film structure in a nano-meter order like a strained layer quantum well structure that is employed in a semiconductor laser element.
Along the development of semiconductor technologies in recent years, it has become possible to easily form a multi-layer film structure in a nano-meter order. As a result, it has become possible to provide a semiconductor laser element having a strained layer quantum well structure, for example, which makes it possible to obtain efficient laser light-emission characteristics.
In the semiconductor laser element having such a strained layer quantum well structure, it is necessary to evaluate actual strain of a lattice. It is possible to evaluate such a strain by measuring a lattice constant. As a method for measuring a lattice constant, generally measurements are performed using a diffraction image based on the Bragg diffraction.
As a method for measuring a lattice constant, there is the X-ray diffraction (XRD) method. In the X-ray diffraction method, X-rays are irradiated onto a specimen to be evaluated, and then a lattice constant of the specimen is obtained by computer simulation based on an X-ray diffraction image diffracted from the rotated specimen. According to this X-ray diffraction method, it is necessary that the specimen at least has an area of 1 mmxc3x971 mm where measurement can be per-formed.
On the other hand, as methods for measuring a lattice constant using the transmission electron microscope (TEM), there are the selected-area electron diffraction (SAD) method (see FIG. 15A), the nano-beam electron diffraction (NBD) method (see FIG. 15B), and the condenser-beam electron diffraction (CBD) method (see FIG. 15C). These methods are used for obtaining an electron diffraction image according to a transmission electron beam that has passed through a specimen by irradiating an electron beam onto the specimen.
In the selected-area electron diffraction method, as shown in FIG. 15A, all electron beams that pass through a condenser lens 133 are irradiated onto a specimen 112 by maintaining these electron beams substantially in parallel, and an electron diffraction image is obtained from a fine area that is limited by a fine hole of a selected-area aperture 137 disposed between an objective lens 135 and an intermediate lens 138. In the selected-area electron diffraction method, the minimum measuring range is 200 nmxcfx86, and the spread of diffraction spots is 10xe2x88x925 to 10xe2x88x926 rad. Further, the measurement precision of a lattice constant to be analyzed is about 3 digits.
In the nano-beam electron diffraction method, as shown in FIG. 15B, a small electron beam of a condenser angle that has been converged to a nano-meter order through the condenser lens 133 and the condenser aperture 134 is irradiated onto the specimen 112, thereby to obtain an electron diffraction image. In the nano-beam electron diffraction method, the minimum measuring range is 2 nmxcfx86, and the spread of diffraction spots is 10xe2x88x923 to 10xe2x88x924 rad. Further, the measurement precision of a lattice constant to be analyzed is about 2 digits.
In the condenser-beam electron diffraction method, as shown in FIG. 15C, a large electron beam of a condenser angle that has been converged to a nano-meter order through the condenser lens 133 and the condenser aperture 134 is irradiated onto the specimen 112, thereby to obtain an electron diffraction image. A computer simulation is carried out based on this electron diffraction image, and a lattice constant of the specimen is obtained based on a result of this simulation. The computer simulation is carried out because the electron diffraction image obtained according to the condenser-beam electron diffraction method becomes a complex diffraction image. In the condenser-beam electron diffraction method, the minimum measuring range is 2 nmxcfx86, and the spread of diffraction spots is 10xe2x88x922 to 10xe2x88x923 rad. Further, as an electron diffraction image having a large volume of information is obtained, the measurement precision of a lattice constant is about 4 digits.
However, in case of measuring a lattice constant of a strained layer quantum well active layer having a multi-layer film structure in a nano-meter order, the measuring range becomes 5 to 500 nm. Therefore, there is a drawback that it is not possible to obtain a sufficient electron diffraction image when the X-ray diffraction method having a minimum measurement range of 1 mmxc3x971 mm or the selected-area electron diffraction method having a minimum measurement range of 200 nmxcfx86 is used, as their measurement range of a specimen is too small. As a result, these methods have had a problem that it is not possible to measure a lattice constant of a strained layer quantum well active layer.
Further, in case of measuring a lattice constant of a strained layer quantum well active layer by using the nano-beam electron diffraction method that has a minimum measurement range of 2 nmxcfx86, the precision of measuring the lattice constant is not sufficiently high. Therefore, there has been a problem that it is not possible to obtain a necessary enough level of precision.
Further, in case of measuring a lattice constant of a strained layer quantum well active layer by using the condenser-beam electron diffraction method, as the minimum measurement range of 2 nmxcfx86, and the measurement precision of a lattice constant is about 4 digits, it is possible to measure in high precision a lattice constant of each layer that constitutes the strained layer quantum well active layer. However, there has been a problem that it is difficult to measure in high precision the lattice constant of the strained layer quantum well active layer in which the lattice constant of each layer has periodicity.
In other words, even if it is possible to measure in high precision the lattice constant of each layer of the strained layer quantum well active layer by using the condenser-beam electron diffraction method, it is difficult to measure the lattice constant of each layer based on the same measuring condition. Particularly, in the strained layer quantum well active layer, each layer has a spatial strain distribution. Therefore, there has been a problem that it is not possible to accurately measure an average strain of the lattice constants based on a simple averaging of lattice constants through a discrete measurement of each layer.
It is an object of the present invention to provide a method of and an apparatus for measuring a lattice constant capable of measuring promptly and in high precision the average value of the lattice constants of layers that form a multi-layer film structure in a nano-meter order like a strained layer quantum well active layer. It is another object of this invention to provide a computer program that contains instructions which when executed on a computer realizes the method according to the present invention on the computer.
The method of measuring a lattice constant according to one aspect of the present invention comprises following processes. That is, a flux of highly parallel electron beams are irradiated onto a specimen having a multi-layer film structure in a nano-meter order. Then, an electron diffraction image diffracted from the specimen is recorded onto a photosensitive member. Finally, the recorded electron diffraction image is analyzed, and a lattice constant of a multi-layer film structure of the specimen is measured based on a result of the analysis.
The apparatus for measuring a lattice constant according to another aspect of the present invention irradiates a flux of electron beams onto a specimen, records an electron diffraction image based on a diffraction of the electron beams passed through the specimen, and analyzes the electron diffraction image and measures a lattice constant of the specimen. Moreover, there is provided a condenser aperture at an electron beam source side of the specimen having a strained layer quantum well structure. With this arrangement, a fine flux of electron beams having a condenser angle of 0.5 mrad or below and having an electron-beam diameter of 20 nm to 100 nm are irradiated onto the specimen.
Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings.